As radio waves are used as the transmission medium in radio communications, level deviation or phase deviation of received signals takes place due to fading or interference to deteriorate transmission characteristics. Diversity reception has been known as an effective method to prevent such deterioration in transmission characteristics. The most typical diversity reception is postdetection diversity receiver which instantaneously selects a fading signal having the highest received signal level out of plural fading signals each of which changes independently. The reception system is based on the assumption that the higher the level of received signals, the higher is the S/N ratio (signal/noise ratio) and the smaller is phase deviation, and thus the transmission quality is expected to improve.
FIG. 1 shows a prior art typical 2-branch postdetection diversity receiver wherein two antennas 101, 102 are installed with a suitable interval distance between them in order to obtain fading waves independently, and are respectively connected to receivers 103, 104. As the detector of the received signal levels, IF log-amplifiers 105, 106 are used for intermediate frequency. The IF log-amplifiers 105, 106 are equipped with a means to output DC voltage proportionate to decibel of the received signal level. By comparing the output voltages from the two log amplifiers 105, 106 by a level comparator 107, the received signal levels can be compared. With the comparator output, output from one of the detectors 108 or 109 with the higher received signal level is selected by a diversity select switch 110. This allows an output terminal 111 to constantly receive detector outputs with a higher S/N ratio and smaller phase deviation.
However, the system requires plural receivers, and the size of the whole system inevitably becomes large. There has been proposed an antenna select diversity reception system as a simpler structure wherein plural antennas are switched to receive signals by only one receiver.
FIG. 2 shows a prior art typical 2-branch antenna select diversity reception circuit wherein two antennas 121 and 122 are placed at a suitable interval distance therebetween so as to obtain fading waves independently (not correlated) from each other. Either one of the antennas 121, 122 is connected to a receiver 124 with an antenna selector 123. The switching is conducted with the comparator data from a level comparator 126. More particularly, the received signal level of the receiver 124 is detected by a received signal level detector 125, which in turn is compared with the reference level by the comparator 126, and when, for example, IF signal envelope of the received signal becomes lower than a predetermined threshold value, the antennas 121 and 122 are switched.
To distinguish from said antenna select diversity reception, the receiver shown in FIG. 1 which switches detection outputs will be referred to as "post-detection diversity" hereinafter.
The following problems will occur if the received signal level is used as the data for branch selection in the postdetection diversity system.
The first problem is that the log-amplifiers 105, 106 are required to have a level detection performance over a wide range of signal levels. In practice, however, log-amplifiers could not quite meet such requirements as their level detection characteristics are often saturated in the extremely high or low levels or deviated from linearity in the intermediate range. In the ranges where the characteristics are saturated or deviated, the difference in output voltages decreases although the received signal levels differ between branches, to thereby hamper accurate comparison of received signal levels. As it is difficult to make level detection characteristics of log-amplifiers agree between branches over a wide range of levels, errors occur in the comparison result in the range where disagreement is excessive to thereby decrease the effect of diversity.
The second problem is that when the branch characteristics deteriorate, the diversity effect decreases. In other words, when distortion increases due to the deterioration caused by chronological changes or improper adjustment in the log-amplifier 108 or the detector 109 of either one of the branches in FIG. 1 and the detection characteristics of the branch is deteriorated, the S/N ratio decreases or phase deviation increases inconveniently if received signal levels alone are compared to switch to the detector 109.
The third problem is that the diversity receiver by means of comparison of received signal levels is ineffective when the transmission characteristics deteriorate due to reasons other than the drop of the level of received signals. For example, considering the effect on the interference on the same channel, the effect from the interference waves is generally less when the received signal levels are high and therefore the diversity effect can be achieved. However, as interference waves also change in level due to fading, C/I (desired signal level/interference signal level) becomes smaller if the received signal level is high as shown in FIG. 3(a) than when said level is low as shown in FIG. 3(b), and the probability of errors may increase. In such a case, even if the diversity reception by comparison of received signal levels is used, desired result cannot be achieved.
The fourth problem lies in the fact that when the levels of the signals received in both of the two branches are low, the received level comparator is actuated by thermal noises and precise comparator outputs cannot be obtained. For example, as shown in FIGS. 4(a), 4(b) and 4(c), even if the levels of the desired signals are substantially equal, the levels differ depending on the noise vector when thermal noises are greater. When diversity reception is conducted with the comparator outputs, the transmission characterisitcs may sometimes be deteriorated.
The antenna select diversity reception is not free of problems and the following problems tend to occur when received signal levels are used as the information for branch selection.
Similarly to the third problem in the postdetection diversity reception, the high levels of the received signals do not necessarily mean better transmission quality than the low levels thereof. In other words, even if the level of received signals is high, an error is sometimes caused while no error may occur at a low level. If selection is made based only with the levels of received signals, signals with errors might be selected.
The second problem arises from the fact that IF log-amplifiers are used to detect received signal levels in the antenna select diversity reception. The first problem mentioned in relation to the postdetection diversity reception will occur in the antenna select diversity reception, too.
The third problem arises when a mean of received signal levels is to be obtained as the output from a received signal level detector. For the purpose, a filter is generally inserted at an output of the detector, but delay inevitably occurs in the level comparator output. Because of the delay, when antennas are switched at a high speed, sufficient diversity effect cannot be achieved.
The fourth problem encountered in the case of post-detection diversity reception will also occur in the antenna selection diversity reception if the levels of the input signals are low in both of the branches.
An object of this invention is to overcome those problems encountered in the prior art and to provide a diversity receiver which can achieve diversity effect without the need to detect the received signal levels and which can achieve a remarkable effect not only against fading but also against all the factors which affect transmission circuits.